The present invention relates to electrical monitoring devices, and more particularly, to a device for monitoring the electrical continuity of a pair of electrical control conductors which extend along a plurality of subway cars and which provide an electrical control signal to each of the cars.
Various types of rail transit cars have individual propulsion and braking systems for each car making up the train. Such cars may, therefore, be operated independently or grouped together in trains. It is necessary to provide a control circuit extending along the length of a train of such cars so that the engineer at the front of the train can control operation of the braking and propulsion systems in each of the cars simultaneously. One system which has been developed for providing such control uses a pair of electrical conductors which extend the entire length of each transit car. Connectors are provided at both ends of each car such that the control conductors may be connected to those of the adjoining cars. A current source is provided at the front of the train for operation by the engineer and a bridging electrical conductor is then placed across the control conductors at the end of the last car in the train. Transducers in each car monitor the current flow through the conductors. A nominal current value, such as 500 milliamps, is applied by the current source to the control conductors. When it is desired to accelerate the train, the current is increased above 500 milliamps; conversely, when braking is desired, the current is reduced below the 500 milliamp level. This control system is clearly essential to operation of the transit cars.
Since all of the conductors are connected in series, if a fault should occur in any conductor or connector, the entire trail will be disabled. The isolation of temporary faults or opens in the control conductor loop presents extremely troublesome problems since after the problem corrects itself, it is often extremely difficult to isolate the cause to a single car. Such momentary faults may be of a recurring nature with the result that the operation of the train will become extremely undependable. It will, therefore, be necessary to remove the entire train from service and spend an excessive amount of time in locating the car in which the intermittent faults occurs.
It is seen, therefore, that a circuit is needed for monitoring operation of the control conductor system and for detecting the occurrence of a fault in the system even if such fault should occur only intermittently and for determining in which of the individual cars the fault occurs. Further, a fault indication should be provided and maintained, even if all power is removed from the system.